1. Field of the Invention
The present invention is concerned with signal transmission tube assemblies, such as coils of shock tube on a spool, which assemblies include sealing means for improved safety in shipping and storage. More specifically, the present invention is concerned with signal transmission tube assemblies having the open or terminal ends thereof sealed to reduce the hazard of unintended ignition and that of contamination of the signal transmission tube.
2. Related Art
The use of signal transmission lines or tubes, such as those commonly referred to-as "shock tube" is, of course, well-known in the art. It is conventional practice in the manufacture of signal transmission tube to coil the tube on large spools which may contain as much as ten thousand linear feet (about 305 meters) of the signal transmission tube. Signal transmission tube, as is well-known, comprises hollow tubing usually made of synthetic organic polymeric materials (plastics) and contains on the interior wall thereof a coating of a reactive material. In the case of shock tube, the reactive material contains a pulverulent high explosive. The coating of reactive material on the interior wall is quite thin and leaves the tube hollow, providing an open channel or bore extending through the length of the shock tube. When the reactive material is initiated as by a spark igniter or any other suitable means, the reaction of the reactive material propagates an initiation signal through the bore of the tube. If the tube is properly connected to a device such as a detonator cap, the signal emerging from a terminal end of the shock tube will initiate the detonator cap. Patents which illustrate the construction and manufacture of shock tube include U.S. Pat. No. 3,590,739 issued Jul. 6, 1971 to P. A. Persson, U.S. Pat. No. 4,328,753 issued May 11, 1982 to L. Kristensen et al and U.S. Pat. No. 4,607,573 issued Aug. 26, 1986 to G. R. Thureson et al. As disclosed in the Thureson et al Patent, the reactive material of a shock tube may comprise a thin coating or dusting of a mixture of high brisance explosive such as PETN, RDX, HMX or the like, and a fine aluminum powder. The tube of the shock tube may be a plural layer tube. For example, as disclosed in the Kristensen et al Patent, the tube may comprise an inner or sub-tube such as a SURLYN ionomer plastic and the outer tube may be made of a mechanically tougher material such as a low or medium density polyethylene. (SURLYN is a trademark of E. I. Du Pont de Nemours & Co. for its ionomer resins.)
U.S. Pat. No. 4,757,764 issued Jul. 19, 1988 to G. R. Thureson et al discloses signal transmission tubes as described above except that the reactive material is a low velocity deflagrating material instead of an explosive powder of high brisance. The use of a deflagrating material reduces the speed of transmission of the initiation signal propagated through the tube as compared to shock tubes. Such deflagrating material tubes are usually referred to in the art as low velocity signal transmission lines or tubes ("LVST tubes"). A large number of deflagrating materials are disclosed in U.S. Pat. No. 4,754,764, including manganese/potassium perchlorate, silicon/red lead and zirconium/ferric oxide, to name but a few of the many compositions disclosed starting at column 3, line 48 of the Patent. As pointed out at column 4, line 47 et seq of that Patent, whereas shock tubes, when ignited, produce a "shock wave initiation signal" which travels through the tube, the LVST tubes transmit an initiation signal by means of a "pressure/flame front" principle. Nonetheless, both types of tubes, shock tubes and LVST tubes, may be utilized as signal transmission lines and both emit a signal from an open, terminal end of the tube.